Wavelength conversion lasers have been developed that convert the wavelength of a fundamental wave laser to converted waves such as second harmonic wave (second harmonic generation: SHG), sum-frequency wave (sum frequency generation: SFG) or difference frequency wave (difference frequency generation: DFG) and the like. Internal cavity type wavelength conversion lasers, in which a wavelength converting element is inserted into a cavity of a solid-state laser, are characterized by enabling highly efficient wavelength conversion as a result of utilizing a cavity structure.
Since solid-state lasers, and particularly microchip solid-state lasers using laser crystals on the sub-millimeter order to the order of several millimeters, are compact and allow the obtaining of W class output, they are expected to be used in various applications. Combinations of microchip solid-state lasers and internal cavity type wavelength conversion lasers have been attempted to be applied in wavelength ranges in which semiconductor lasers are unable to oscillate directly and fields requiring giant pulses or high frequencies. They are expected to be applied in the fields of imaging and analysis in particular.
However, in the case of applying to imaging or illumination fields and the like, internal cavity type wavelength conversion lasers have the problems of having a narrow spectral width and the generation of dotted pattern interference noise referred to as speckle noise. In addition, internal cavity type wavelength conversion lasers also required the solid-state laser to have a single mode and narrow band in order to enable highly efficient wavelength conversion.
Internal cavity type sum frequency generation has been previously proposed that uses two types of solid-state laser crystals. For example, Patent Document 1 proposes combining the resonance optics of laser beams oscillating at different wavelengths from two types of solid-state laser media, and generating sum-frequency wave of the combined two-wavelength beam with a frequency converting element. In addition, Patent Document 2 proposes configuring a dual-wavelength cavity that shares a single reflecting mirror using two types of solid-state laser crystals and three reflecting mirrors, and carrying out sum-frequency mixing with a non-linear optical crystal.
In addition, since wavelength conversion lasers that carry out multi-longitudinal mode oscillation have reduced speckle noise and other interference noise in comparison with single longitudinal mode lasers, they can be applied in imaging and illumination fields. In internal cavity type wavelength conversion lasers, in the case of carrying out multi-mode oscillation with a solid-state laser, there is the problem of a decrease in conversion efficiency. During high output of a solid-state laser in particular, multi-mode oscillation progresses resulting in a decrease in conversion efficiency. In addition, in internal cavity type wavelength conversion lasers, wavelength conversion efficiency cannot be freely set to a high level in the manner of the transmittance of a laser exit window since wavelength conversion efficiency is typically low. Consequently, in order to increase the efficiency of internal cavity type wavelength conversion lasers, it is necessary to reduce internal loss of materials within the cavity, thereby also resulting in the problem of increased material costs.
In addition, in the case of microchip solid-state lasers, since the laser crystals are pumped on the sub-millimeter order, pumping light of the semiconductor laser focuses on the laser crystal. Since microchip solid-state lasers have a short absorption length for the laser crystal pumping light, those locations of the laser crystals that generate heat become concentrated, thereby decreasing efficiency at high output and causing the problem of instability. Consequently, temperature regulation is required for stable operation thereby resulting in handling difficulties. In addition, internal cavity type wavelength conversion lasers generate extremely high levels of mode competition noise referred to as a green problem that occurs due to insertion of a non-linear optical crystal into a laser cavity that carries out multi-mode oscillation, thereby resulting in the problem of unstable output.
Patent Document 3 proposes a configuration for realizing a stable, multi-mode, internal cavity type wavelength conversion laser in which divided pumping light is radiated onto a laser crystal, and laser light having multiple optical axes is oscillated with a single cavity to generate second harmonic waves of each optical axis of the laser light. This configuration is described to be resistant to external disturbances and demonstrate stability as a result of having laser light of multiple independent optical axes.
However, although the configurations of internal cavity type sum-frequency wave generation using two types of solid-state laser crystals as proposed in the prior art described above allow sum-frequency generation, they are not able to carry out other waveform conversion. Consequently, waveform converted light of multiple wavelengths cannot be obtained. In addition, mode competition noise, which is thought to occur during high output and during multiple wavelength oscillation, is not taken into consideration. Moreover, since the waveform conversion efficiency of internal cavity type waveform conversion lasers it typically low, high waveform conversion efficiency cannot be freely set in the manner of the transmittance of a laser exit window. Consequently, in order to increase the efficiency of internal cavity type wavelength conversion lasers, it is necessary to reduce internal loss generated from materials within the cavity, thereby resulting in the problem of increased material costs.
In addition, since conventional multi-mode internal cavity type wavelength conversion lasers as described above carry out wavelength conversion independently with each optical axis by oscillating laser light having multiple optical axes, there is the problem of decreased conversion efficiency as compared with wavelength conversion with a single optical axis. In addition, since it is necessary to have numerous optical axes in order to obtain stability, handling of the outgoing beams becomes difficult. In addition, it is also necessary to control temperature.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-279739    Patent Document 2: Japanese Patent Application Laid-open No. 2006-66436    Patent Document 3: Japanese Patent Application Laid-open No. 2007-73552